BISS (Basic Interoperable Scrambling System)
Learn about it at http://www.scientificatlanta.com/products/customers/white-papers/BISS.pdf
You see it used on PPV special events. The BISS receiver allows the operator to input a key to allow the reception of the PPV. Used extensively on the special events. Scientific Atlanta's PowerVu is commonly used but if you are thinking of buying one, their price is extremely high and they normally don't sell this type of receiver to the public. Providers will overnight the receiver to the event host and the receiver is returned after the event.
Alternatives? DMS International makes BISS receivers that are actually affordable. We sell these to event providers that need the ease and security of operator activation.